JERUSALEM — Israel's fastest-in-the world vaccine campaign, which reached half its citizens as of Sunday, is offering other countries the first real-life look at how mass inoculation can bend the trajectory of the coronavirus pandemic.

A rush of Israeli medical research — some emerging too fast for academic journals to keep up — reveals that the Pfizer-BioNTech vaccine is even more effective than hoped at preventing serious disease and death, safe for nearly all adults, and almost completely free of serious side effects.

Israel’s scientific results have allowed health officials to open the inoculation program to pregnant women and nursing mothers, while the findings also showed that it was safe for those with food allergies and autoimmune diseases.

And the vaccine has proved so good at protecting the elderly, who are especially vulnerable, that hospital administrators here say it has all but eliminated the risk that covid-19 cases would collapse their critical care systems. Almost 90 percent of Israelis over 50 have been fully vaccinated.

Israel’s small population of about 9 million, and its universal national health system, makes it a natural vaccine lab for the world. The program has inoculated more than 4.6 million people with at least the first injection, the fastest per capita pace of any country. More than 3.3 million have gotten both shots.

Pfizer has guaranteed a steady supply of vaccine doses in exchange for access to anonymized data from the digitized medical file kept on almost every Israeli. This data has allowed scientists from universities, the health ministry and Pfizer to track the vaccine’s impact with unprecedented speed.

“Sometimes when you go from clinical trials to the real world, you get different results,” said Hagai Levine, an epidemiologist at Hebrew University and chairman of the Israel Association of Public Health Physicians. “What these studies are showing us is the vaccine gives very good protection against disease and mortality. That is great news for the world.”

But the research has yet to answer some urgent questions, including how well the vaccine performs against emerging variants of the virus. And experts are split on what the studies show about how contagious people remain after they are vaccinated, a key issue in slowing the infection’s spread.

Nor has Israel yet become the example the world craves of life returning to normal. The country’s rate of infection remains stubbornly high, apparently driven by the arrival of variant strains and the failure of some communities to comply with coronavirus restrictions. Vaccination rates among ultra-Orthodox Jews and some Israeli Arab neighborhoods remain below average. (In Palestinian territories captured by Israel in the 1967 war, the vast majority of the population has yet to be inoculated, and Israel has faced criticism at home and abroad for not providing more vaccines.)

Still, the run of upbeat reports has reassured health officials who watched anxiously as the new vaccines were deployed within months, instead of the years that vaccine development has usually taken.

Within weeks of Israel giving its first vaccinations in late December, snap studies were obtained by the media, tweeted by their authors and publicized by research institutions. Some scholars cautioned that the good news had yet to be peer reviewed, but officials and pandemic-weary people worldwide, desperate for information, have devoured the findings.

“It’s science on steroids,” said Eran Segal, a professor at the Weizmann Institute of Science.

Segal tweeted the results of one of the first significant studies in early February as the oldest Israelis — the first to get shots — completed the two-shot regimen. His team found a 41 percent drop in new covid cases among those over 60.

Publication of that study is waiting on journal editors, but other scientific results — and the experience of hospital administrators — have confirmed that fewer elderly people are flooding into the covid wards.

“That is the most important finding in Israel so far — that when it comes to protecting the health-care system, the vaccine is winning,” said Ronni Gamzu, Israel’s former coronavirus “czar” and the head of Tel Aviv’s Sourasky Medical Center, one of the country’s largest hospitals.

The number of covid patients in his facility has declined by two-thirds in two months, Gamzu said. “You may still have a high infection rate, but once you vaccinate 90 percent of people over 50, you are abolishing the real threat of the epidemic,” he said.

Another major study, conducted jointly by Israel’s Ministry of Health and Pfizer and reported by the Israeli news site Ynet last week, showed a 93 percent plunge in serious covid disease and death among the vaccinated.

The study, yet to be peer-reviewed, also reported that inoculation cut transmission of the virus by 90 percent, a finding hailed by many scientists as a vital pathway to herd immunity. But other experts were skeptical, noting the difficulty in detecting infections that may show no symptoms, and suggested that further surveillance of cases was needed.

To determine the vaccine’s impact across the population, health experts are parsing Israel’s trove of data by age, gender, location and health histories going back 20 years.

Analysts can create virtual control groups so it’s possible to compare hundreds of thousands of vaccinated people with unvaccinated people who share the same profiles.

“We individually matched every single vaccinated person to their, if you want, unvaccinated twin,” said Ran Balicer, head of the research institute of Clalit, the largest of Israel’s four public HMOs. “You can do this only when you have near-perfect data sets.”

For instance, if the vaccine has been given to a 56-year-old ultra-Orthodox Jewish man from Tel Aviv with two underlying conditions, Balicer said, researchers would match him with an unvaccinated 56-year-old ultra-Orthodox man from the same neighborhood and with the same conditions.

Researchers from Clalit, Harvard University and Boston Children’s Hospital compared 1.2 million Israelis split into two groups. They found two shots of the vaccine to be 94 percent effective in preventing symptomatic illness, a potentially pandemic-ending success rate if enough people are inoculated.

Other data dives quickly offered preliminary answers to questions that the clinical trials did not. A study of 9,000 staffers at Sheeba Medical Center in Tel Aviv, for example, showed the Pfizer vaccine to reduce symptomatic cases by 85 percent after only one dose.

Israel’s rapid vaccine deployment is being closely watched around the world, as other nations grapple with their own uneven rollouts amid growing public frustration over health restrictions.

One takeaway from the early findings in Israel is clear, said Zoe McLaren, an associate professor of public policy at the University of Maryland at Baltimore County: “Pour crazy amounts of money into a vaccination program, because it’s going to end the pandemic.”

That said, there are major differences between Israel and other countries, and the quantity of vaccine doses is one. In many nations, including the United States, the rollout has been partly hampered by insufficient supply. Israel’s small size and centralized health care also limit the usefulness of comparisons.

The data from Israel “shows what you can do when you have an intact health system and a simplified, streamlined health system like Israel’s,” said Peter J. Hotez, a vaccine and infectious diseases expert at Baylor College of Medicine. The experience in Israel is reason for hope, he said, but with new virus variants spreading, “it means we’ve got to move faster.”

Even in Israel itself, vaccination seems to be racing against an increasingly daunting foe.

Officials are loosening Israel’s third national lockdown and have launched a “green passport” system that grants the vaccinated special access to concerts, gyms and other once-routine parts of life.

But airports, borders and some schools remain closed. And as younger people have become eligible for the vaccine — anyone 16 and older can now get a free jab — the pace of inoculations has slowed. Epidemiologists warn that Israel could be far from done with masks and curfews.

“We are taking control of the pandemic,” Levine said. “But taking control is not elimination. We are not there yet.”

Brulliard reported from Washington.